MIMO (multiple-input multiple-output) refers to a method for improving transmission/reception efficiency by adopting multiple transmit (Tx) antennas and multiple receive (Rx) antennas rather than using a single Tx antenna and a single Rx antenna. That is, MIMO is a technology for increasing capacity or improving performance by using multiple antennas at a transmitting end or a receiving end of a wireless communication system. MIMO may be referred to as multi-antenna technology. To correctly perform multi-antenna transmission, it is necessary to feed back information on a channel from a receiving end that receives multiple antenna channels.
Information fed back from a receiving end to a transmitting end in a conventional multi-antenna wireless communication system may include a rank indicator (RI), a precoding matrix indicator (PMI), a channel quality indicator (CQI), etc., which may be collectively referred to as channel state information (CSI). The CSI is composed of information suitable for conventional multi-antenna transmission. In general, RI may be determined as a value indicating a most suitable rank in the current channel state, PMI may be determined as a value indicating a most suitable precoding matrix based on the rank, and CQI may be determined as a value indicating a most suitable modulation and coding scheme (MCS) based on the rank and the precoding matrix.